1. Field of the Invention
The present invention is related to a method and a device for evaluating quality of concrete structures. The device includes a sensing film able to detect the time when a source of waves is generated. With the device, stress waves are generated and the time that the source of waves is generated is recorded. So that the evaluation work which conventionally requires dual receivers only needs a single receiver for completion of the work.
2. Description of the Prior Art
Conventionally, nondestructive testing techniques for concrete have been developed, wherein, an impact-echo method developed in the middle of 1980 generates stress waves in concrete by impact; the degree of the impact force and the size of the device for impact can be adjusted to control the desired energy and frequency of the stress waves. The impact-echo method uses a signal receiver which is made of inverted conical piezo-electric material, the device makes a point contact with concrete, so that the surface of the concrete needs no grinding for flattening. Application of the impact-echo method in measuring the thickness of concrete plates has been included in the ASTM as a standard test method in 1998.
A time-of-flight diffraction technique for detecting the depth of a surface-opening crack in concrete uses steel spheres as impact sources. After impact on the concrete, longitudinal waves (P waves: pressure waves), transverse waves (S waves: shear waves) and Raleigh""s waves (R waves: surface waves) are generated. Wherein, the P waves and S waves are propagated into concrete in the shape of a semi-sphere, while R waves are spread out from the impact point on the concrete surface in the shape of a circle. The source of the waves is generated at the moment when the sphere strikes the surface of the concrete; by virtue that the time when the sphere impacts the surface of the concrete to generate the source of the waves can not be obtained directly, an indirect way was used conventionally to determine the occurring time of the source of waves by locating a receiver close to the impact point. The principle of the indirect way for detecting the vertical depth of a surface-opening crack is as below:
The longitudinal waves (P waves) and the transverse waves (S waves) generated by impacting the surface of the concrete propagate inwardly of the concrete; the P waves go faster, hence the wavefront thereof meets the tip of the crack firstly, then the S waves arrive, the incident P waves generate diffraction waves at the tip of the crack to propagate in the shape of a sphere as a new source of waves generated at the tip of the crack. When the diffraction waves are transmitted back to the impacted surface of the concrete, disturbance is induced. To record the time for stress waves traveling from the impact point via the diffraction at the crack tip to arrival at the surface on the other side of the crack, two receivers sensitive to the vertical displacement of particles are respectively disposed at both sides of the surface-opening crack. The waveform of the displacement detected by the receiver at the same side as that of the position of impact is controlled mainly by a downward displacement created by the R waves; thereafter, the waveform is influenced by the disturbances caused by arrivals of the reflecting waves and the diffraction waves. The initial disturbance detected by the receiver at the different side from that of the position of impact is caused by the arrival of the P wave diffracted from the tip of the crack, because the crack that impedes and delays arrival of the R waves. And then, the displacement waveform detected is generated by the subsequent arrivals of the reflecting waves and the diffraction waves.
FIG. 1 is a schematic view showing evaluation and detection of the crack. A first receiver 93 is located at a distance HO from the source of impact 91, the distances between the source of impact 91 and the crack and between the crack and a second receiver 94 are respectively H1 and H2. When the first receiver 93 receives a downward displacement created by the R waves, the signal monitoring system is activated. We hereby provide that arrival time of the R waves is t1, the time that the second receiver 94 records arrival of the diffraction waves going around the tip of the crack is t2, and the time difference between sensing the arrival of the R waves by the first receiver 93 and sensing the arrival of the diffraction waves by the second receiver 94 is t2 t1.
Impact is done before sensing the arrival of the R waves by the first receiver 93, thereby, the initial time of impact shall be obtained by derivation. And this is the propagation time of the R waves from the source of impact 91 to the first receiver 93, i.e., the value of HO divided by the speed of the R waves (CR). Thereby, the time period of the P waves from the source of impact 91 to the second receiver 94 can be obtained from the following calculation formula:                               Δ          ⁢                      xe2x80x83                    ⁢          t                =                              t            2                    -                      t            1                    +                                    H              o                                      C              R                                                          (        1        )            
The length of path that the P waves go along equals to the multiplication of the speed of the P waves (CP) and the time taken, thereby, the depth (d) of the surface-opening crack in concrete is calculated according to the following formula:                     d        =                                                            [                                                                                                    (                                                                              C                            P                                                    xc3x97                          Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          t                                                )                                            2                                        +                                          H                      1                      2                                        -                                          H                      2                      2                                                                            2                    xc3x97                                          C                      P                                        xc3x97                    Δ                    ⁢                                          xe2x80x83                                        ⁢                    t                                                  ]                            2                        -                          H              1              2                                                          (        2        )            
Analysis of Difficulty Resided in the Prior Art
The prior art has five steps in detecting a crack, it is hereby stated as follows (referring to FIG.1):
Step 1 (measuring speed of the R waves CR): The two receivers 93, 94 are allocated on the surface 92 of concrete and has a distance therebetween S, impact 91 is done at a position a suitable distance from the first receiver 93, the vertical displacement waveform detected shows evident arrival of the R waves. From the detected waveform, the times of arrival of the R waves at the two receivers 93, 94 are TR1 and TR2 respectively, thereby, the time period that the R waves propagated from the first receiver 93 to the second receiver 94 is xcex94TR=TR2xe2x88x92TR1 then the speed of the R waves is obtained, i.e., CR=S/xcex94TR.
Step 2 (measuring speed of the P waves CP): Before arrival of the R waves, there has been displacement disturbance induced by the arrival of the P waves; however, amplitude of vibration of it is smaller than that of the disturbance induced by the arrival of the R waves. If the front portion of the waveform thereof is partially amplified, the displacement response caused by the arrivals of the P waves at the first receiver 93 and the second receiver 94 respectively are TP1 and TP2. Similar to the step 1, the speed of the P waves is obtained, i.e., CP=S/TP2xe2x88x92TP1.
Step 3 (determining HO, H1 and H2): After the speed of the R waves and the speed of the P waves are obtained-, detecting of the crack is performed in site, H0, H1 and H2 are given, the displacement waveform obtained by the first receiver 93 allocated at the same side of the source of impact shows the initiating time t1 of downward disturbance induced by the arrival of the R waves; the second receiver 94 is allocated at the opposite side of the source of impact 91, the first arriving wave sensed by the second receiver 94 is a diffraction wave going around the tip of the crack, the waveform thereof shows the arrival time t2 of the diffraction wave.
Step 4 (substituting in the formula (1) to obtain the value of At): Deriving from the formula (1), we get the time when an impact is done, and get the time period of propagation xcex94t that the P waves arrive at the second receiver 94 diffracted via the tip of the crack. Step 5 (substituting in the formula (2).to obtain the value of the depth d): The time period of propagation and the speed of the P waves are substituted in the formula (2) to obtain the depth d of the crack.
Examples show that, although the time-of-flight diffraction technique leading in the source of waves by impact can accurately detect the depth of the crack on concrete, it has the following defects:
1. The R waves must be detected in the first place before derivation of the occurring time of the source of waves: Calculation for this is complicated, and can increase difficulty and variation in evaluation; thereby, efficiency of evaluation is lowered.
2. The technique can not be operated by only one person: When in operating, a person can not maintain the two receivers in positions simultaneously and has the sphere struck on a predetermined position accurately; normally an assistant is wanted, that is, the two persons shall operate together.
3. The technique requires a dual receiver; this makes higher complexity of equipment arrangement and cost.
The principal object of the present invention is to provide an evaluation method for concrete structures; it can conveniently and accurately detect the depth of a surface-opening crack in concrete. Another object of the present invention is to provide an evaluation method for concrete structures, wherein, the work of evaluation can be completed only with a single receiver; this not only reduces cost, but also simplifies the works of evaluation and signal analysis.
The present invention will be apparent after reading the detailed description of the preferred embodiment thereof in reference to the accompanying drawings, wherein: